To protect a single thyristor or any of the individual thyristors of a thyristor valve (i.e. an array of thyristors connected in series) against overvoltages caused by excessive thyristor valve voltage, by improper voltage shearing, or by the failure of individual firing control channels or signals, a backup firing circuit is used which fires an individual thyristor when its off-state voltage reaches a preset level. One conventional type of backup firing circuit uses a single breakover diode. Here the breakover diode connects the anode of the thyristor with its gate through an appropriate impedance when the applied voltage across the thyristor reaches the tripping level of the breakover diode. The resulting gate current pulse fires the thyristor. Another approach is to use a special firing circuit to initiate the gate firing pulses whenever the thyristor voltage monitored by an individual sensing circuit reaches the level required for the protective action. With these approaches the triggering voltage level to which the individual emergency firing is preset is close to the maximum voltage ratings of the thyristor. Lowering the emergency firing level would cause the protection circuit to interfere with the operation of the valve during normal overvoltage transients.
In the event of sustained overvoltage conditions or on the sustained loss of the gate control signal to an individual thyristor, the individual emergency firing circuit will be in continuous operation as long as the thyristor switch is on. When the emergency firing voltage trigger level set to a high level (up to 90% of the thyristor voltage rating) the capacitor in the snubber circuit normally associated with each thyristor will be charged to higher voltage levels and the energy dissipation in the subber resistor during continuous emergency firing will be much higher than that corresponding to normal operating conditions. The continuous turn-on with the maximum snubber discharge current can also reduce the operating life of the thyristor. Additional problems may arise due to poorer controllability of the thyristor valve where a faulty gate firing channel is present. Instead of starting precisely at the instant of the gate firing signal, the conduction of the thyristor valve may be delayed until the thyristor valve voltage reaches the level set for the emergency firing of the device having a faulty gate firing channel. In the specific case of a static VAR compensator, this can result in excessive generation of harmonics in a thyristor-switched capacitor bank or in an undesirable dc offset in a thyristor-controlled reactor bank.
Conventional emergency firing circuits use values for the emergency triggering levels that are high with respect to the rate values of the thyristors for reasons of efficiency and economy of operation. However, it would be advantageous to have a circuit that would allow emergency firing at high levels yet reduce the stress in the thyristor should repeated firing of the thyristors under emergency condition be required. It is one object of the invention to provide initial firing of the thyristor at a high emergency triggering level approaching the voltage rating of the thyristor while providing a reduced triggering voltage level protection subsequent to the initial emergency triggering condition. This allows both economy of operation under normal operating conditions and protection of the individual thyristor and its associated snubber circuit during prolonged emergency firing conditions.